Inorganic ions are an essential requirement for life and are found in large amounts in drinking water, blood and every cell of an organism as well as in the environment. For example, the concentration of many ions i.e. sodium, potassium, magnesium, and calcium inside and outside of cells is essential for any living organism. Consequently, the ion concentration in the blood and blood cells of animals and human beings also is of high importance for a large variety of body functions.
Normally lithium is a trace element present in blood plasma. Lithium is also used as a drug to treat bipolar mood disorder. It is estimated that worldwide over one million people take lithium on a daily basis. A disadvantage in the use of lithium is the very low therapeutic index, i.e. the ratio between the toxic concentration and the therapeutic concentration. Most patients respond well to a blood plasma concentration of 0.4-1.2 mmol/L lithium while toxic effects can occur at a lithium concentration of above 1.6 mmol/L. A prolonged high blood lithium level can even result in permanent damage to the nervous system and even death. Monitoring of the lithium concentration during treatment is therefore essential, with regular checks every couple of months to keep the lithium level at desired level.
To avoid extensive operator handling, ion-selective electrodes (ISEs) are routinely used for measurements of blood parameters in an automated fashion. These ISEs are fast and offer a large dynamic range. However, their response is logarithmic and the required high selectivity for lithium can be a problem. Additionally, in case of lithium intoxication a fast procedure for blood analysis is required. Currently, a venous blood sample must be withdrawn from the patient by specially trained personnel and transported to the central laboratory and the blood cells need to be removed before the measurement is made. This procedure can take up to 45 minutes. To minimize sample throughput time and enable measurements on location, miniaturized devices employing ion-sensitive field-effect transistors are available to determine the concentration of potassium and sodium in whole blood even as a hand-held analyzer. However, such analyzers are not used for lithium determination, because of the high background concentration of other charged species, in particular sodium ions, compared to the much smaller concentration of lithium ions.
The direct measurement of lithium in whole blood and the determination of inorganic cations in blood plasma have been described and demonstrated by E. Vrouwe et al. in Electrophoresis 2004, 25, 1660-1667 and in Electrophoresis 2005, 26, 3032-3042. Using microchip capillary electrophoresis (CE) with defined sample loading and applying the principles of column coupling, a concentration of alkali metals was determined in a drop of whole blood. The whole blood collected from a finger stick was transferred onto a microchip without extraction or removal of components of the whole blood. The lithium concentration can be determined in the blood plasma from a patient on lithium therapy without sample pre-treatment. Using the microchip with conductivity detection, a detection limit of 0.1 mmol/L has been obtained for lithium in a 140-mmol/L sodium matrix.
Other prior art documents disclosing several types of the microchips for the measurement of the concentration of ions in the blood sample are known in the art. For example, US Patent Application US 2005-0150766 (Manz) discloses a capillary electrophoresis microchip.
U.S. Pat. No. 5,882,496 (Northrup et al) discloses a method for fabrication and use of porous silicon structures to increase a surface area of one of electrophoresis devices.
U.S. Pat. No. 7,250,096 (Shoji et al, assigned to Hitachi High-Technologies Corp) teaches a method and apparatus for measuring a current-carrying path during electrophoresis to detect the state of the current-carrying path.
One of the issues in the prior art is a formation of gas bubbles in the electrolyte at the electrodes (as noted in U.S. Pat. No. 7,250,096) and/or undesired redox (reduction-oxidation) reaction due to electrolysis at electrodes in a microchannel of the apparatus. This occurs because the charge transport through the apparatus is carried by electrons in an electric path and ions in a chemical path. The charge is exchanged between electrodes and ions at the electrodes.
The electrolyte in the microchannel has a specific gas capacity. The maximum amount of the specific gas capacity is termed the gas limit. The gas bubbles are formed when the gas limit is reached locally within the microchannel. The formation of gas bubbles directly influences the measurements.
The ions and other uncharged molecules undergo changes due to redox reactions and changing concentrations at the electrodes. The gas bubbles are formed due to the formation of non-charged molecules which exceed the gas limit and form gas bubbles. These gas bubbles are confined within the microchannel of the device and as a result can distort the measurements.
The formation of the gas bubbles can be avoided as is explained in prior art if there is a single electrical circuit for capillary electrophoresis measurement or a single electrical circuit for in contact conductivity detection and voltage and or current is controlled adequately. However, if there are two electrical circuits for the measurement method combined, then the electrical interference of both electrical circuits adds complications.
Prior art methods of resolving this problem for single electrical circuits include the use of alternating current between the electrodes, by limiting of the electrical current, by controlling the type of redox reaction and by reducing the voltage below the redox potential. Limiting the current can for instance be realized by using a current source, small channel geometries and low concentrations of the electrolyte in a channel. It is also possible to use a low concentration of background electrolyte in a channel. Furthermore the design of the electrodes can play a role. Electrodes with large surface area are less susceptible to the formation of gas bubbles since the charge concentration changes are spread over a larger area.